The French composer-conductor could hardly have chosen a more dramatic means of heralding his return to Chicago, after a 17-year absence, as well as the local debut of his Ensemble InterContemporain from IRCAM, the experimental lab for acoustical and musical research that he heads in Paris.

Boulez has nearly always tended to write music for ears and minds further advanced on an evolutionary scale than those of the majority of his new-music audience, or so it has seemed from some of his earlier, more recondite works. But ``Repons`` (response) in many ways reflects a far more mellow, unabashedly romantic Boulez, delighting in the coloristic interchange of acoustic and electronic sound as perhaps only a modern French composer of modern French sensibilities can do.

It is easier to describe the physical layout of ``Repons`` than the strange and rather wonderful hold its kaleidoscopic play of sonorities exerts on the listener. On a stage at the center of the gym were Boulez, his 24-member chamber ensemble and the technicians who operated the computer-synthesizers. The audience encircled the stage. Around the periphery of the room were six soloists playing amplified pianos, harp, celesta, cimbalom, xylophone, vibraphone, electric organ and percussion. It was these sounds that were electronically altered and that swept through the audience via loudspeakers hung throughout the gym.

For all the attention the composer drew to those computer-manipulations in his talk to Northwestern students the previous evening, ``Repons`` contains very little of the weird sonic exoticism one associates with most electronic music. Most of the timbres within the 45-minute piece, in fact, sound like normal keyboard and percussion timbres amplified and played back ``canned.``

Not until the final section for soloists alone, when ``Repons``--its fierce rhythmic energies largely spent--winds down into quiet, terse statements separated by silences, was this listener fully aware of any overt electronic transformation.

That element aside, one must report that musically, sonically and spatially ``Repons`` is perhaps the most unabashedly sensuous score Boulez has written. His fascination with Oriental music, particularly swirling, gamelan- like sounds suspended in space, is as telling as his repeated references (an hommage of brightly clangorous piano and percussion) to his teacher, Olivier Messiaen.

Sometimes the mind--this listener`s mind, at any rate--balked at processing the sheer amount and density of eruptive detail. Repeated listenings would help to put more of the musical events in perspective. But

--Boulez being Boulez--even the most event-laden textures are uncannily lucid, a tribute as much to the pristine clarity of his scoring as to the precision of his conducting. Boulez the maestro, in fact, kept the entire apparatus together with his usual coolly heroic efficiency, while his virtuosi di IRCAM threw themselves into their duties with remarkable urgency and even passion.

``Repons`` may not be the seminal breakthrough in acoustic-cum-electronic sound that Boulez envisions. Nor is it even complete as it stands in this fourth (1985) version of what is expected to be a two-part, evening-long piece. But what is to be heard is nevertheless provocative and appealing, and it was in that adventuresome spirit that the attentive audience embraced the event. A second program by Boulez and his IRCAM forces will take place Friday at Orchestra Hall. No listener who cares about musical culture in the late 20th Century can afford to stay away.